Circuit configurations to reduce snapback of a transient voltage suppressor

ABSTRACT

This invention discloses a transient voltage suppressing (TVS) circuit that includes a triggering Zener diode connected between an emitter and a collector of a bipolar-junction transistor (BJT) wherein the Zener diode having a reverse breakdown voltage BV less than or equal to a BVceo of the BJT where BVceo stands for a collector to emitter breakdown voltage with base left open. The TVS circuit further includes a rectifier connected in parallel to the BJT for triggering a rectified current through the rectifier for further limiting an increase of a reverse blocking voltage. The triggering Zener diode, the BJT and the rectifier are formed in a semiconductor substrate by implanting and configuring dopant regions of a first and a second conductivity types in an N-well and a P-well whereby the TVS can be formed in parallel as part of the manufacturing processes of the electronic device.

This Patent application is a Continued Prosecution Application (CPA) ofa co-pending application Ser. No. 13/066,907 filed by common inventorsof this Application on Apr. 26, 2011. Application Ser. No. 13/066,907 isa Divisional Application and claims the Priority Date of an applicationSer. No. 11/444,555 filed on May 31, 2006 now issued as U.S. Pat. No.7,538,997, and application Ser. No. 12/454,333 filed May 15, 2009 on bycommon Inventors of this Application. The Disclosures made in the patentapplication Ser. Nos. 13/066,907, 11/444,555 and 12/454,333 are herebyincorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to a circuit configuration and method ofmanufacture of a transient voltage suppressor (TVS). More particularly,this invention relates to an improved circuit configuration and methodof manufacture of a transient voltage suppressor (TVS) with greatlyreduced snapback.

2. Description of the Relevant Art

The transient voltage suppressors (TVS) are commonly applied forprotecting integrated circuits from damages due to the inadvertentoccurrence of an over voltage imposed onto the integrated circuit. Anintegrated circuit is designed to operate over a normal range ofvoltages. However, in situations such as electrostatic discharge (ESD),electrical fast transients and lightning, an unexpected and anuncontrollable high voltage may accidentally strike onto the circuit.The TVS devices are required to serve the protection functions tocircumvent the damages that are likely to occur to the integratedcircuits when such over voltage conditions occur. As increasing numberof devices are implemented with the integrated circuits that arevulnerable to over voltage damages, demands for TVS protection are alsoincreased. Exemplary applications of TVS can be found in the USB powerand data line protection, Digital video interface, high speed Ethernet,Notebook computers, monitors and flat panel displays.

FIG. 1A-1 shows a typical commercially available two-channel TVS array10. There are two sets of steering diodes, i.e., diodes 15-H and 15-Land 20-H and 20-L respectively for each of the two input/output (I/Os)terminals I/O-1 and I/O-2. Furthermore, there is a Zener diode, i.e.,diode 30, with a larger size to function as an avalanche diode from thehigh voltage terminal, i.e., terminal Vcc, to the ground voltageterminal, i.e., terminal Gnd. At a time when a positive voltage strikeson one of the I/O pads, the high side diodes 15-H and 20-H provide aforward bias and are clamped by the large Vcc-Gnd diodes, e.g., theZener diode 30. The steering diodes 15-H and 15-L and 20-H and 20-L aredesigned with a small size to reduce the I/O capacitance and therebyreducing the insertion loss in high-speed lines such as fast Ethernetapplications. FIG. 1A-2 shows the reverse current IR versus reverseblocking voltage characteristics of the two-channel between the Vcc andthe ground voltage of the TVS 10 shown in FIG. 1A-1. The reverse currentIR as that shown in the diagram of FIG. 1A-2 represents a reversecurrent conducted through the Zener diode, i.e., between Vcc and GND.Here it is assumed that the reverse BV of each steering diode is higherthan the reverse BV of the Zener diode. But note that at high currentswhen the Vcc to Gnd pad voltage is equal or higher than the summation ofthe reverse BV of the steering diodes then the current would also flowthrough all the two series steering diode paths. Since the Zener diodehas higher resistance per unit area compared with BJT or SCR and BJTthis is actually a disadvantage at higher currents because the steeringdiodes also have to be rugged in reverse conduction. In the case of theSCR+BJT the Zener clamp voltage is lower at higher currents and hencethe steering diodes paths will not conduct. The breakdown voltage of theVcc-Gnd diode 30 and the steering diodes 15 and 20 should be greaterthan the operating voltage (Vrwm) so that these diodes only turn-onduring the voltage transients. The problem with the Vcc-Gnd clamp diodesis that typically these diodes are very resistive in reverse blockingmode and require large area to reduce resistance. As shown in FIG. 1A-2,the high resistance leads to the increase of BV at high current. This isnot desirable as high BV not only causes the break down of steeringdiodes as described above but also causes damage to the circuit the TVSdevice intends to protect. The requirement to have large diode size thuslimits further miniaturization of a device when such TVS circuit isimplemented.

One common method used in the integrated circuits to circumvent thisdrawback is to use a Zener triggered NPN as the clamp device as thatshown in FIG. 1B-1. The TVS circuit 50 shown in FIG. 1B-1 comprises aNPN bipolar transistor 55 connected in parallel to a Zener diode 60 tofunction as a Zener triggered NPN bipolar TVS device. FIG. 1B-2 shows acurrent-voltage (IV) diagram for the Zener triggered NPN diode device.FIG. 1B-2 illustrates that when the collector voltage of the NPN 55reaches the breakdown voltage of the Zener diode 60, the NPN bipolarturns-on and snaps back to a lower voltage called the BVceo or holdingvoltage where BVceo stands for collector to emitter breakdown voltagewith base left open. However, in a device that implements a TVS circuit,the snap-back phenomenon is not desirable. The snap-back creates asudden drop of the reverse voltage that often causes the circuitoscillations due to negative resistance.

Therefore, a need still exists in the fields of circuit design anddevice manufactures for providing a new and improved circuitconfiguration and manufacturing method to resolve the above-discusseddifficulties. Specifically, a need still exists to provide new andimproved TVS circuits that can perform good voltage clamping function,occupying smaller areas and eliminating or reducing snapback voltagevariations.

SUMMARY OF THE PRESENT INVENTION

It is therefore an aspect of the present invention to provide animproved TVS circuit to have an improved clamping. It is therefore afurther aspect of the present invention to provide an improved TVScircuit to reduce the voltage-drop in a reverse-blocking voltagesnap-back when a reverse current transmitted over a Zener diode triggersand turns on an NPN bipolar transistor. The TVS circuits disclosed inthis invention thus resolve the difficulties caused by increasing breakdown voltage due to TVS device resistance and drastic voltage drop dueto a snapback that commonly occurs in the conventional TVS circuit.

Moreover, it is another aspect of the present invention to provide animproved device design and manufacturing method to provide an improvedTVS circuit. Specifically, most commercially available TVS aremanufactured using a discrete process or older bipolar technology.However, this new TVS can be integrated into mainstream CMOS or Bi-CMOStechnology allowing future single chip TVS protected ICs. Integrationcomes with lower cost protected ICs.

Briefly in a preferred embodiment this invention discloses a transientvoltage suppressing (TVS) circuit for suppressing a transient voltage.The transient voltage suppressing (TVS) circuit includes a triggeringdiode, such as a Zener diode, connected between an emitter and acollector of a first bipolar-junction transistor (BJT) wherein the Zenerdiode having a reverse breakdown voltage BV less than or equal to aBVceo of the BJT where BVceo stands for a collector to emitter breakdownvoltage with base left open. The TVS further includes a second BJTconfigured with the first BJT to function as a silicon controlledrectifier (SCR) wherein the first BJT triggers a SCR current to transmitthrough the SCR for further limiting an increase of a reverse blockingvoltage caused by a transient voltage. In an exemplary preferredembodiment, the first BJT further includes a NPN bipolar junctiontransistor (BJT). In another preferred embodiment, the triggering diodeand the BJT with the SCR are integrated as a semiconductor integratedcircuit (IC) chip. In another preferred embodiment, the Zener diodetriggering the first BJT for transmitting a current through the firstBJT in a BJT mode and turning on the SCR at a higher reverse currentthan an initial current transmitting through the first BJT.

In another preferred embodiment, the present invention further disclosesan electronic device formed as an integrated circuit (IC) wherein theelectronic device further includes a transient voltage suppressing (TVS)circuit. The TVS circuit includes a triggering diode connected betweenan emitter and a collector of a first bipolar-junction transistor (BJT)wherein the triggering diode having a reverse breakdown voltage BV lessthan or equal to a BVceo of the first BJT where BVceo stands for acollector to emitter breakdown voltage with base left open. The TVScircuit further includes a second BJT connected in parallel to the firstBJT forming a SCR for conducting current through the SCR for furtherlimiting an increase of a reverse blocking voltage. In a preferredembodiment, the triggering diode, the first BJT and the SCR are formedin a semiconductor substrate by implanting and configuring dopantregions of a first and a second conductivity types in an N-well and aP-well whereby the TVS can be formed in parallel as part of themanufacturing processes of the electronic device.

The present invention further discloses a method for manufacturing anelectronic device with an integrated transient voltage suppressing (TVS)circuit. The method includes a step of connecting a triggering diodebetween an emitter and a collector of a first bipolar-junctiontransistor (BJT) with the triggering diode having a reverse breakdownvoltage BV less than or equal to a BVceo of the first BJT where BVceostands for a collector to emitter breakdown voltage with base left open.The method further includes a step of connecting a second BJT inparallel to the first BJT for SCR function to conduct current throughthe SCR for further limiting an increase of a reverse blocking voltage.In a preferred embodiment, the step of connecting the SCR furthercomprising a step of connecting a second silicon controlled rectifier(SCR) anode for conducting a SCR current when triggered for currentconduction at higher reverse current

These and other objects and advantages of the present invention will nodoubt become obvious to those of ordinary skill in the art after havingread the following detailed description of the preferred embodiment,which is illustrated in the various drawing figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A-1 is a circuit diagram for showing a conventional TVS device andFIG. 1A-2 is an I-V diagram, i.e., a current versus voltage diagram, forillustrating the reverse characteristics of the TVS device.

FIG. 1B-1 is a circuit diagram for showing another conventional TVSdevice and FIG. 1B-2 is an I-V diagram for illustrating the reversecharacteristics of the TVS device with the voltage presents a suddensnap-back voltage drop at the time when a current conduction over theNPN bipolar transistor is triggered.

FIG. 2A is a circuit diagram for showing a TVS circuit of this inventionand FIG. 2B is an I-V diagram for illustrating the reversecharacteristics of the TVS device with significantly reduced snap backvoltage drops.

FIGS. 3A to 3D are side cross sectional views of circuit components ofthe TVS device of FIG. 2A supported on a semiconductor substrate for alow side diode, a high side diode and a clamp diode respectively.

FIGS. 3E to 3G are top views of the TVS device for the device shown inFIGS. 3A, 3C and 3D respectively where FIGS. 3A and 3B are crosssectional view along lines A-A′ and B-B′ respectively.

FIG. 4A is a circuit diagram for showing a TVS circuit implemented witha clamp diode with auxiliary P+ anode and FIG. 4B is a cross sectionview of the TVS circuit of FIG. 4A supported on a semiconductor device.

FIG. 5 is a cross sectional view of the TVS device shown in FIG. 4A.

DETAILED DESCRIPTION OF THE METHOD

Referring to FIGS. 2A and 2B for a circuit diagram and an I-V diagram,i.e., a current versus voltage diagram, respectively of a TVS circuit100 of this invention. The TVS circuit 100 is installed between a groundvoltage terminal (Gnd) 105 and a Vcc voltage terminal 110 to function asa Vcc-Gnd clamp circuit. The TVS circuit 100 includes two sets ofsteering diodes, i.e., diodes 115-H and 115-L and 120-H and 120-Lrespectively for each of the two input/output (I/Os) terminals 125-1 and125-2. Furthermore, there is a Zener diode, i.e., diode 130, with alarger size to function as an avalanche diode from the high voltageterminal, i.e., terminal Vcc, to the ground voltage terminal, i.e.,terminal Gnd. The Zener diode 130 is connected in series with a resistor135 and in parallel to a NPN bipolar transistor 140. A PNP bipolartransistor 142 in configured with NPN bipolar transistor 140 forms aPNPN silicon-controlled rectifier (SCR) structure 150 with high holdingcurrent and voltage. The breakdown voltage, i.e., BV, of the triggeringdiode 130 is less than or equal to the BVceo of the NPN bipolartransistor 140 where BVceo stands for collector to emitter breakdownvoltage with the base left open.

BV (Trigger Diode)≦BVceo

FIG. 2B is a current versus voltage diagram for comparing theoperational characteristics of the TVS according to FIG. 2A and theconventional TVS. As a transient voltage higher than a normal operatingvoltage is applied to the TVS circuit, a reverse current is triggered topass through the Zener diode 130 because the breakdown voltage BV of thetrigger diode 130 is adjust to less than the BVceo. As the voltageincreases, the device migrates into BJT mode where the NPN 140 conducts.When the voltage increases further the SCR 150 is activated and beginsto conduct current. The turning on of the SCR causes a slight drop ofthe reverse blocking voltage VR. FIG. 2B also shows the I-V diagrams,i.e., curve 160 for diode TVS of FIG. 1A-1, and curve 170 for BJT TVS ofFIG. 1B-1. In contrast to curves 160 and 170, by adjusting BV of triggerdiode less than the

0 BVceo, the voltage snap back problems are resolved. The sequence ofoperation mode provides the benefit of fast response as the NPNtransistor turns on fast. Furthermore, by turning on the SCR 150 at aSCR trigger voltage to protect the NPN bipolar transistor 140, theincrease of BV at high current is minimized as the SCR action incurs theleast resistance. This resolves the difficulties of high BV at highcurrent that not only causes the break down of steering diodes but alsocause damage to the circuit the TVS device intends to protect.

The detail operation of the TVS system can be further understood fromthe following descriptions. Typically the TVS is biased in a system withthe high voltage terminal Vcc and the ground voltage Gnd connected tothe system which needs protection. There are also applications where theVcc is left floating for specific applications. Then a +Ve or −Ve zap isapplied to the I/O terminals with respect to Gnd. When a +Ve zap isapplied on I/O the upper diodes are forward biased and when the voltagereaches the trigger diode BV then current flows through the resistor 135in series with the trigger diode 130. When the drop in the resistor 135reaches a voltage of 0.6V then the base-emitter junction of the NPNtransistor 140 is forward biased and the NPN transistor 140 turns-on.Now the collector current of the NPN transistor flows through theresistor connected between the emitter and base of the PNP transistor142. When the potential drop in this resistor 145 reaches a voltage of0.6V then the emitter of the PNP transistor 142 begins to conduct andthe SCR action is initiated. So now the current flows from the anode onthe PNP 142 transistor, i.e., the emitter of PNP, to the cathode of theNPN transistor 140, i.e., the emitter of the NPN. On the negative zapthe bottom diode turns-on in forward conduction between I/O pad and Gndand ESD current flows only in this diode path. There is also a conditionwhen a voltage zap is applied to the Vcc at a voltage of +Ve withrespect to Gnd. Under this zap condition the current flows through theVcc-Gnd path, i.e., there is no current conducted in the steeringdiodes, since the trigger diode breaks down and initiates the SCR asdescribed above.

FIGS. 3A to 3D are a cross sectional views showing the typical TVS arraythat includes the improved trigger diode 130 integrated with NPN bipolartransistor 140 and the PNP bipolar transistor 142 forming SCR 150, andtwo sets of steering diodes 115-L, 115-H, and 120-L, 120-H. The new TVSarray 100 as shown in FIG. 3A to is manufactured with mainstream CMOStechnology. FIGS. 3A and 3B show a TVS array supported on a P substrate200. A P type region 210 is placed next to an N+ region 215 forming aZener diode 130 with cathode connecting to Vcc pad 110. A P+ region 220also connects to Vcc pad 110. P+ region 220 disposed next to an N wellregion 230 above P substrate 200 forms PNP transistor 142, with Psubstrate 200 connecting to Gnd pad 105 through P well 240 and P region242. The lateral path in P substrate 200 from N well 230 to P well 240provides the resistance for resistor 135. The path from N region 235 toN well 230 provides resistance 145. The N-well 230 disposed above theP-substrate 200 in turn electrically contacting an N-region 245 thusconstituting the NPN transistor 140. The P-type region 210 formed nextto the N+ region 215 within P well 240 is to tailor the triggerbreakdown voltage BV of the trigger diode 130, i.e., the diode formedbetween the P region 210 and the N+ region 215, to be less than or equalto the BVceo of the NPN transistor 140. The other way of tailoring theBV and BVceo is to increase the gradient of the N doping of N+ region235 so that the collector to emitter breakdown voltage with the baseleft open (CEO) is tailored to the desired value. A combination of thetwo could also be used to get desired BV and BVceo.

FIG. 3C shows the low side steering diode comprises a P+ region 280 andN+ region 285 encompassed in a P-well 290. FIG. 3D shows the high sidesteering diode comprises a P+ region 280′ and N÷ region 285′ encompassedin a N-well 290′. For lowering the capacitance of these diodes and alsoincreasing their BV, a lower doped N− region is added where N+ region isimplanted so the process provides an N+/N−/PW diode instead of N+/PWdiode. Similar for the high side diode, a P− implantation is added whereP+ is so as to give a P+/P−/Nwell diode.

FIG. 3E is a top view of the TVS device according to the configurationsshown in FIGS. 3A to 3D. The N+ and P+ diffusion regions 215 and 220 inFIG. 3A are masked by the active region. The NW 230 below the N+ regions215 is connected to Gnd 105 that increases the base resistance of theNPN transistor and also helps to turn-on the SCR at high currents. ThePT regions 210 used as anode of the trigger diode are staggered in thelayout and cut line A-A′ and B-B′ cross-sections are shown in FIGS. 3Aand 3B respectively. The P+ region 220 of the SCR anode region is alsostaggered in the layout to control the SCR holding current. The NW 230under the P+ emitter 210 or anode forms the collector of the NPNtransistor that forms part of the SCR. The top view layout of the lowside and high side diodes are shown in FIGS. 3F and 3G. The N+/NW guardrings 260 and the P+/PW guard rings 270 are formed to suppress latch-upduring ESD transients between I/Os and I/O to VCC.

FIGS. 4A and 4B are circuit diagram and I-V diagram respectively forshowing an alternate embodiment with further improved clampcapabilities. The TVS system shown in FIG. 4A has similar circuitconfiguration as that shown in FIG. 2A except with two SCR anodes 150-1and 150-2. By integrating multiple SCRs anode structures as shown inFIG. 4A provides the improved current handling and clamping capabilityas that shown in FIG. 4B. A cross-section of the multiple integratedSCRs anode structure is shown in FIG. 5. The operational principles andcircuit connections are similar to that of FIG. 2A. Briefly, a transientvoltage breaks down trigger diode 130. When the voltage drop at resistor135 reaches 0.6 V the NPN transistor 140 turns on and current flowsthrough resistors 145-1 and 145-2. When the combined voltage drop overresistors 145-1 and 145-2 reaches 0.6 V the first anode of SCR 150-1 isinitiated. When the SCR current continue increase to the point thevoltage drop over resistor 145-2 reaches 0.6 V the second anode of SCRkicks in. The number of SCR anodes can be increased to meet the need ofprotection circuit. The benefit of multiple SCR anodes provides theadvantage that when each SCR anode kicks in, their correspondingsnack-back forces the locking voltage close to the maximum clampingvoltage therefore provides an improved clamping.

According to FIGS. 3 and 4, this invention discloses an electronicdevice with the triggering diode and the SCR are integrated with theelectronic device on a single chip. In a preferred embodiment, thetriggering diode and the SCR are manufactured with a standard CMOStechnology and integrated with the electronic device on a single chip.In another embodiment, the triggering diode and the SCR are manufacturedwith a standard Bi-CMOS technology and integrated with the electronicdevice on a single chip. In another embodiment, the TVS further includesa second SCR anode connected in parallel to a first SCR anode of the SCRand the first BJT triggering a SCR current at a higher reverse currentfor conducting the reverse current through the second SCR anode forfurther limiting an increase of a reverse blocking voltage. In anotherembodiment, the TVS further includes guard ring for suppressing alatch-up during an ESD transients between an I/O pad to a high voltageVcc terminal. In another embodiment, the triggering diode, the SCR areformed in a semiconductor substrate by implanting and configuring dopantregions of a first and a second conductivity types in an N-well and aP-well whereby the TVS can be formed in parallel as part of themanufacturing processes of the electronic device.

With the above circuit diagrams and the device cross sections, theinvention shows the TVS operations and array integration of the improvedTVS devices. These TVS devices provide improved clamp protections thatoccupy smaller area and perform good clamping function because the SCRsare able to carry high currents with little voltage drop beyond triggerdiode breakdown.

Although the present invention has been described in terms of thepresently preferred embodiment, it is to be understood that suchdisclosure is not to be interpreted as limiting. Various alterations andmodifications will no doubt become apparent to those skilled in the artafter reading the above disclosure. Accordingly, it is intended that theappended claims be interpreted as covering all alterations andmodifications as fall within the true spirit and scope of the invention.

1. A method for manufacturing an electronic device with an integratedtransient voltage suppressing (TVS) circuit comprising: connecting atriggering diode between an emitter and a collector of a firstbipolar-junction transistor (BJT) with said triggering diode having areverse breakdown voltage BV less than or equal to a BVceo of said firstBJT where BVceo stands for a collector to emitter breakdown voltage withbase left open; connecting a second BJT with said first BJT forming aSCR whereas said first BJT triggering a SCR current through said SCR forfurther limiting an increase of a reverse blocking voltage; andtriggering said first BJT by said triggering diode for transmitting acurrent through said first BJT in a BJT mode by turning on said SCR at ahigher reverse current than an initial current transmitting through saidfirst BJT.
 2. The method of claim 1 further comprising: said step ofconnecting said triggering diode between an emitter and a collector of abipolar-junction transistor (BJT) comprising a step of connecting aZener diode to a bipolar junction transistor (BJT).
 3. The method ofclaim 1 wherein: said step of connecting said triggering diode betweenan emitter and a collector of a bipolar-junction transistor (BJT)comprising a step of connecting said diode to a NPN bipolar junctiontransistor (BJT).
 4. The method of claim 1 wherein: said step ofconnecting said triggering diode and said SCR is a step of forming saidtriggering diode and said SCR on an integrated circuit (IC) chip.
 5. Amethod for manufacturing an electronic device with an integratedtransient voltage suppressing (TVS) circuit comprising: connecting atriggering diode between an emitter and a collector of a firstbipolar-junction transistor (BJT) with said triggering diode having areverse breakdown voltage BV less than or equal to a BVceo of said firstBJT where BVceo stands for a collector to emitter breakdown voltage withbase left open; connecting a second BJT with said first BJT forming aSCR whereas said first BJT triggering a SCR current through said SCR forfurther limiting an increase of a reverse blocking voltage; andconnecting at least a steering diode in parallel to said triggeringdiode for electrically connecting to an input pad for steering a normalcurrent between a high voltage and a low voltage terminal.
 6. The methodof claim 5 further comprising: integrating said triggering diode andsaid SCR with said electronic device on a single chip.
 7. The method ofclaim 5 further comprising: manufacturing said triggering diode and saidSCR with a standard CMOS technology and integrated with said electronicdevice on a single chip.
 8. The method of claim 5 further comprising:manufacturing said triggering diode and said SCR with a standard Bi-CMOStechnology and integrated with said electronic device on a single chip.9. The method of claim 5 further comprising: tailored said reversebreakdown voltage BV of said triggering diode.
 10. The method of claim 5further comprising: suppressing a latch-up during an ESD transientbetween an I/O pad to a high voltage Vcc terminal of said TVS byproviding a guard ring for said TVS.
 11. The method of claim 5 furthercomprising: connecting a triggering Zener diode to said BJT and saidrectifier further comprising a step of forming in a semiconductorsubstrate by implanting and configuring dopant regions of a first and asecond conductivity types in a N-well and a P-well whereby said TVS canbe formed in parallel as part of the manufacturing processes of saidelectronic device.